Technique for producing interconnecting conductive links

ABSTRACT

A method for providing a lateral conductive link between conductive elements, e.g., metals, placed on a first non-conductive material, wherein a second non-conductive material is placed on said first non-conductive material to form an interface therebetween in a region between the conductive elements. Energy is applied to the conductive elements to produce mechanical strains by thermal expansion in the conductive elements which initiate a rupturing of the interface between the non-conductive materials so as to provide at least one fissure therein extending between the conductive elements. The energy applied causes a portion of at least one of the conductive elements to flow in such fissure to provide a lateral conductive link between the conductive elements.

GOVERNMENT FUNDING

This invention was made with government support under Contract NumberF19628-90-C-002 awarded by the Air Force. The government has certainrights in the invention.

INTRODUCTION

This invention relates generally to techniques for producinginterconnecting conductive links and, more particularly, to a uniqueprocess for providing conductive links between two conductive materialshaving a non-conductive material positioned between them.

BACKGROUND OF THE INVENTION

Configurations of interconnected arrays of conductive elements, as used,for example, in programmable logic gate arrays, requires the formationof conductive links, or paths, between selected conductive elements in amanner which produces relatively low resistance links between them.Techniques for producing such low resistance conductive links have beendeveloped using either electrical or laser linking and cuttingprocesses.

The latter laser processes have been preferred for certain applicationsbecause they provide permanent links and require no programming wiringor high voltage switching on the chip. Laser programmable techniqueshave the potential for providing higher performance and greater linkdensity than electrical techniques if the linking device itself issufficiently small. Ultimately the minimum size laser link would be asimple crossing of two wires. However, up to now insofar as is known, asuccessful process does not exist for providing such links. A primaryconcern when using any linking technology is the ability to use standardprocessing for the metal lines on the insulation. More specifically,this means the ability to integrate laser restructurable elements usingstandard silicon based MOS processing without the need to incorporateadditional steps. Fuse links, which produce conductive links usingsilicon diffusion, have been used for some time to achieve compatibilitywith CMOS processing, as disclosed in U.S. Pat. No. 4,937,475, issued toF. M. Rhodes et al. on Jun. 26, 1990.

Other recent exemplary techniques have been proposed using laser linkingprocesses for interconnecting metal layers at different levels. One suchtechnique is disclosed in U.S. Pat. No. 5,166,556 issued on Nov. 24,1992 to F. Shu et al. in which a laser beam is applied to an uppertitanium metal layer at the location at which a link is desired to bemade with a lower titanium layer. Laser power is supplied at asufficient level to cause a chemical reduction reaction between thetitanium layers and the intermediate silicon dioxide insulating layer soas to produce a conductive compound between the titanium layers whichacts as an electrically conductive circuit therebetween. Such techniquerequires additional non-standard process steps and produces highresistance links and, hence, low performance.

U.S. Pat. No. 4,810,663 issued to J. I. Raffell et al. on Mar. 7, 1989discusses a technique in which a diffusion barrier layer is placedbetween each metal layer and the insulation layer and the link region isexposed to a low power laser for a relatively long time (i.e., arelatively long pulse width) to cause the metals to alloy with thediffusion and insulating layers to form the desired conductive link.Such technique requires a relatively long laser power pulse output usinga relatively complicated diffusion barrier/insulation structure so as toproduce an opening in the upper layer to permit the energy to be appliedto the barrier and insulating layers to produce the desired alloyingoperation.

A further technique has been proposed to provide lateral links betweenmetals substantially at the same surface or plane as discussed in U.S.Pat. No. 4,636,404 issued to J. I. Raffell et al. on Jan. 13, 1987.Again relatively long pulses are applied to the general region betweenthe metals so as to cause the metals to form an aluminum doped siliconlink.

In a recent article "Laserpersonalization of Interconnection Arrays forHybrid ASIC's" of M. Burnus et al., IEEE International Conference onWafer Scale Integration, 1993, a laser beam is used to providesufficient power to blast a hole through an upper metal layer so as toform an opening at the link region. Multiple laser pulses of high energydensity are used to create the opening and to remove the insulatinglayer between the metal elements. The multiple pulses also producemolten aluminum which spreads along the walls of a crater that is formedwhen the insulating layer is removed beneath the opening. Such aluminumflow along the crater walls produces a tube-like aluminum contact bodybetween the upper and lower aluminum layers.

The article "Laser Programmable Vias for Reconfiguration of IntegratedCircuits" by Rouillon-Martin et al. in Optical Microlithography andMetrology for Microcircuit Fabrication, 1989, discloses a techniquewhich performs a similar operation to that discussed in the aboveBurnass et al. article in which the opening is made much smaller indiameter by using multiple pulses of a relatively highly focused laserbeam.

It is desirable to devise a laser linking process which produces a linkstructure between any two metal layers which can be fabricated in amanner which is compatible with standard MOS processes and whichprovides high performance (low resistance) and high density (small area)links. Such process should use relatively low laser power and provideself-contained links with low peripheral damage at the link sites.

BRIEF SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, in a linking processan energy producing device, e.g., a laser, applies a single pulse ofsufficient energy to at least one of two conductive materials which areto be linked, and which have a non-conductive material between them, soas to produce mechanical strain in at least one of the conductivematerials. The strain that is produced initiates a fracturing of thenon-conductive material so as to provide at least one fissure thereinwhich extends between the conductive materials. The single energy pulseapplied by the energy producing device further causes a portion of atleast one of the conductive materials to flow in the at least onefissure to provide at least one conductive link between the conductivematerials. In most cases, an effective fissure extends from a point ator near an edge of at least one of the conductive materials to the otherconductive material.

In another embodiment of the invention, an upper metal layer isdeposited on the non-conductive material in a manner so as to provide apre-formed opening at the desired link site. A single pulse of energycan then be used to be effectively applied to the lower metal layer atthe link site so as to produce the mechanical strain required toinitiate the fracturing of the dielectric or insulating material, asdiscussed above. In a still further embodiment of the invention, if apre-formed opening is used in the upper metal layer, a single laserpulse of energy may be used to provide a desired chemical reaction, ordesired alloying, or a desired removal of the dielectric to create acrater therein, without having to produce an opening through the uppermetal layer. Accordingly, by the use of such a pre-formed opening aconductive link may be formed from a chemical reduction reactionprocess, an alloying process, or from the flow of metal in a craterformed in the dielectric which has been removed at the link site. Thus,in some cases the use of the pre-formed opening may not require afracturing of the dielectric material between the metal elements.

DESCRIPTION OF THE INVENTION

The invention can be described in more detail with the help of theaccompanying drawings wherein

FIG. 1 shows a plan view of an embodiment of the invention depicting anexemplary site at which a conductive link is to be produced;

FIG. 2 shows a view in section along the lines 2--2 of the link siteshown in FIG. 1;

FIG. 3 shows a view in section of another embodiment of the inventiondepicting a site at which a conductive link has been produced inaccordance with the invention;

FIG. 4 shows a view in section of another embodiment of the inventiondepicting a site at which a lateral conductive link is to be producedbetween metal elements, generally in the same plane;

FIG. 5 shows a view in section of the site of FIG. 4 in which aconductive link is provided in accordance with the invention;

FIG. 6 shows a view in section of another embodiment of the inventiondepicting a site using an alloying or chemical reaction in thedielectric between the metal layers thereof;

FIG. 7 shows a view in section of another embodiment of the inventiondepicting a site at which a conductive link is provided by forming acrater in the dielectric material between the metal layers thereof; and

FIG. 8 shows a view in section of another embodiment of the invention inwhich a lateral conductive link is to be provided;

FIG. 9 shows a view in section of the embodiment of FIG. 8 in which alateral link has been provided;

FIG. 10 shows a view in section of another embodiment of the inventionin which a lateral conductive link is to be provided;

FIG. 11 shows a view in section of the embodiment of FIG. 9 in which alateral link has been provided;

FIG. 12 shows a view in section of another embodiment of the inventionin which a lateral conductive link is to be provided; and

FIG. 13 shows a view in section of the embodiment of FIG. 12 in which alateral link has been provided.

As can be seen in FIGS. 1 and 2, a lower conductive material in the formof a metal element 10 lies generally in a first plane 11 at a lowerlevel with reference to an upper conductive material in the form of ametal element 12 which lies in generally a second plane 13 above metalelement 10. In the particular embodiment depicted, a non-conductive(insulating) material 14, such as a glass dielectric, effectivelyencloses metal elements 10 and 12 and provides a layer 14A thereofbetween such elements. In a preferred embodiment of the process of theinvention a pre-formed opening 15 is provided in metal element 12 at thedesired link site. The metal elements and the insulating material aremounted on a suitable substrate 16 such as a further glass or otherappropriate substrate material. It is desired to provide an electricallyconductive link between the metal elements 10 and 12.

A link site shown in FIG. 3 is similar to that depicted in FIGS. 1 and2, except that the width of the lower metal layer 10 is co-extensivewith the width of upper metal layer 12.

As shown in FIG. 3 an energy producing device, such as a laser (notspecifically shown in the Figure), is positioned above the link site soas to apply laser energy, depicted by arrows 17, through the structureof FIG. 3 at the opening 15 thereof. Such laser energy is applied in theform of a single pulse of energy, as discussed in more detail below. Theenergy of the single pulse so applied is at a sufficient power levelthat it causes either or both metal elements 10 and 12 to become heatedso that mechanical strains are produced therein and both elementsthermally expand at portions thereof where the energy is applied, asshown by the exaggerated thermally expanded portions 10A and 12Athereof.

The mechanical strains thereby initiate a fracturing of the glassdielectric material 14 so as to produce one or more fissures 18 therein,particularly in the region 14A thereof. While the fissures which are soproduced may extend throughout the material 14 one or more of them willextend from metal element 12 to metal element 10, normally one or moreof the latter fissures extending from a position at or near at least oneedge 19 of metal element 12, as shown by exemplary fissure 18A. Thelaser energy further causes metal from at least one of the metal layers,e.g., the layer with the lower melting point, to flow from such layerinto the fissure so as to contact the other layer, as shown. Similarfissures and at least one conductive link between layers 10 and 12 wouldbe produced in the structure of FIGS. 1 and 2 as shown by exemplaryconductive links 18A in FIG. 2.

In a particular embodiment of the invention, used for the link siteshown in FIG. 3, for example, a continuous wave argon ion laser wasapplied to the link site, which had a site area of 4 μm×4 μm and anopening area of 2 μm×2 μm, via a suitable electro-optical shutter,shutter driver circuitry, and associated optical elements, suchstructures and operation being well known to those in the art. The metallayers were separated by a layer of a silicon oxide dielectric having alayer thickness of 0.75 μm. between the metal layers. The laser producedsingle pulses of laser output power of about 1.0 watts having a pulsewidth from about 1 microsecond (μs) to about 3 μs. The shutter wasarranged to provide laser power rise and fall times, i.e., between about10% to 90% of full power, of about 200 nanoseconds (ns). The laseroutput pulse was formed to provide a minimum beam diameter, through amicroscope objective lens producing a 1/e radius of about 1.0micrometers (μm).

Single pulses of such pulse widths were found to provide a highprobability of producing one or more satisfactory conductive links,i.e., a failure rate of less than one in approximately fifty links, inthe above embodiment. Further increases in the pulse width did notappear to provide any significant improvement in link probability andpulse widths between about 1 to 3 microseconds appeared to be adequatefor the structure shown. In some cases it is believed that even smallerpulse widths can be used, e.g., greater than about 1 nanoseconds (ns) solong as they provide sufficient energy to cause the fracturing and metalflow required.

Moreover, in the above described particular embodiment process it wasfound that using more than a single pulse also did not appear to improvethe probability of forming one or more conductive links and that, if asingle pulse failed to produce at least one link, the use of additionalpulses did not tend to do so.

In the particular embodiment discussed above it was also found thatlaser output power in a range from about 0.5 to 1.0 watts proved to besufficient to provide one or more fissures to form the desiredconducting links. For example, a peak laser output power of 0.72 wattsyielded 98.2% out of 1021 link attempts using pulse widths of onemicrosecond (μs). The differences in linking probabilities, when thepower used was varied from 0.6 to 0.72 watts, for example, using pulsewidths of 2 μs and 3 μs, appeared to be relatively insignificant. It isbelieved that further improvement will occur by scaling theconfiguration of FIGS. 1 and 2, for example, to provide a link site areaof 4 μm×4 μm, or smaller, such that a failure rate of less than 1 in10000 links should be achievable.

The above dielectric fracturing technique for providing vertical linksbetween upper and lower metal layers can also be used to producehorizontal, or lateral, links between metal elements lying insubstantially the same plane, as shown in FIGS. 4 and 5. As seentherein, a pair of separated metal elements 20 and 21 are enclosed in adielectric (e.g., a silicon based glass or polymer) material 22. A laserenergy pulse 25 is applied to the region 23 between the metals. Thelaser pulse produces mechanical strains in the metal elements so thatthey tend to expand so as to provide stresses concentrated at thecorners thereof adjacent region 23, as shown by expanded regions 20A and21A. Such expansions initiate a fracturing of the dielectric material 22so as to produce one or more fissures therein, at least one of whichwill extend from one metal element 20 to the other metal element 21, asshown by exemplary fissures 24. Metal from at least one of the metalswill tend to flow through the at least one fissure to form a conductivelink between metals 20 and 21, as shown by exemplary fissure 24A.

Another embodiment of the invention can also be used for forminghorizontal, or lateral, links between metal elements lying insubstantially the same plane,, as shown in FIGS. 8 and 9. As seentherein, a pair of separated metal elements 40 and 41 are positioned ona first dielectric (e.g., a silicon oxide glass or polymer) material 42and embedded in a second dielectric (e.g., a silicon nitride glass orpolymer) material 43 above the first dielectric material 42.Accordingly, an interface is formed at the adjacent surfaces of thedielectric materials a portion 44 of the interface being formed betweenthe metal elements 40 and 41. Energy in the form of one or more laserpulses 45, for example, is applied generally to the region 46 betweenthe metal elements 40 and 41 particularly so as to be applied to theends of the metal elements.

The mechanical strength characteristics of the dielectric materials aredifferent and the thermal expansion coefficients of the metal elementsare relatively higher than the thermal expansion coefficients of thedielectric materials. Accordingly, when such energy is applied, thermalenergy is absorbed in the metal elements which then expand and producemechanical stresses therein which tend to be concentrated at stressconcentration points, e.g., at the lower corners thereof, at region 46.

As shown in FIG. 9, since the dielectric materials do not expandsignificantly, the expansions of the metal elements initiate a rupturingat the interface 44 of the dielectric materials which separate at theinterface so as to produce a fissure which extends from one metalelement 40 to the other metal element 41, as shown by the exemplaryfissure 47. Molten metal from at least one of the metals flows throughthe fissure 47 to form a lateral conductive link between metals 40 and41.

In a particular embodiment such a lateral link was formed, betweenaluminum elements, in accordance with the technique of the inventiondiscussed with reference to FIGS. 8 and 9, using a diode-pumped Q-switchlaser, such as those made and sold by Spectra Physics Laser DiodeSystems, Inc. of Mountainview, Calif. under the Model 7000 Seriesdesignation as an energy supplying source. The laser provided pulseenergy at about 400 nJ which resulted in a range of usable linkingresistances from about 0.5 ohms to about 5.5 ohms was achieved in theformation of over 1000 links between aluminum alloy elements separatedby about 1.0 μm.

While aluminum, or alloys thereof, are found to be effective for use asthe metal elements involved, any other metal materials having relativelyhigh coefficients of thermal expansion, e.g., copper or copper alloys,can also be used.

While the use of two dielectric materials having different mechanicalstrength characteristics has been discussed in the above embodiment, thesame dielectric material can be used for dielectric layers 42 and 43, solong as the materials are deposited in layers so that a distinct,interface is clearly formed therebetween. Even when the same dielectricmaterials are so used, a rupturing will tend to occur at the interface44 so as to form the desired lateral fissure.

While the dielectric interface is shown in FIGS. 8 and 9 as beingadjacent the lower surfaces of the metal elements, the interface canalso be arranged to be adjacent the upper surfaces of the metal elementsor somewhere in between such upper and lower surfaces. Rupturing and theformation of a fissure can also occur in such latter structures toprovide the desired lateral conductive link between metal elements 40and 41.

Moreover, while FIGS. 8 and 9 show the use of two dielectric materialsto form a single interface, a pair of interfaces 44 and 49 can be formedusing three layers of dielectric materials, as shown in the embodimentof FIGS. 10 and 11. The dielectrics can be different from each other orcan be substantially the same, so long as two distinct interfaces areclearly formed at region 46. In such an approach the application oflaser energy may cause a fissure to be formed at one interface beforeone is formed at the other interface. For example, a fissure 50 can beformed at upper interface 49 from the stress concentration points at theupper corners of the metal elements, prior to the formation of a fissurefrom the stress concentration points at lower interface 44, as shown inFIG. 11. Accordingly, fissure 50 will produce a conductive link betweenmetal elements 40 and 41 in substantially the same manner as discussedabove with reference to the formation of fissure 47.

A further embodiment is shown in FIGS. 12 and 13 wherein, instead offorming a dielectric interface adjacent the upper surfaces of the metalelements, as in FIG. 10, the dielectric material 43 is deposited ondielectric 48 at a level above such upper surfaces to form an interface51 at such level. When energy is applied generally to region 46 andparticularly to the opposing ends of metal elements 40 and 41,mechanical stresses are produced at the stress concentration points,e.g., at the upper corners of the metal elements, which stressesinitiating a fracturing of the dielectric material 48 which produces afissure 52 extending from the upper corners up to the level of theinterface 51 so as to be effectively confined to the region at and belowinterface 51. A rupturing at such interface also tends to occur tofurther enhance the formation of the fissure 52 particularly at and nearthe region of the interface 51. Accordingly, metal flows into suchfissure to form a lateral conductive link between metal elements 40 and41.

In another embodiment of the invention, it has been found that the useof a pre-formed opening, such as pre-formed in an upper metal layer,will permit the formation of a conductive link between an upper metallayer and a lower metal layer without necessarily requiring a fracturingof the dielectric material therebetween. The use of such a pre-formedopening tends to focus the laser beam energy more efficiently at thelink region. As shown in FIG. 6, an upper metal element 30 and a lowermetal element 31 are enclosed by a dielectric material 32, the overallstructure being positioned on a dielectric substrate 33. An opening 34is pre-formed in metal element 30 prior to the application of any energythereto. A single pulse of laser power 35 applied at the region of theopening 34 can be used to cause the metal near such region to flow intothe dielectric region and to alloy with the dielectric material or toproduce a chemical reduction reaction with the dielectric material, asdiscussed above, at the region 36. Such alloying or chemical reactionprocesses cause the dielectric material to become conductive and form aconductive link between metals 30 and 31. Because a preformed opening isused, a single pulse of relatively low laser power can be used incontrast with prior art systems where the laser energy must besufficient to provide the alloying or chemical reaction processesrequired, which processes tend to result in producing an opening in theupper metal element 30.

Alternatively, if an alloying or chemical reaction process is not used,the use of such pre-formed opening permits the laser energy to befocused more effectively at the dielectric material so as to remove suchmaterial in the region under the opening to expose the laser metal layerto form a crater 37. The laser energy also causes the metal fromelements 30 and 31 to flow along the sides of the crater that is soformed, as shown in FIG. 7.

While the particular embodiments of the invention discussed above arepreferred, modifications thereto may occur to those in the art withinthe spirit and scope of the invention. Hence, the invention is not to beconstrued as limited to the specific embodiments discussed, except asdefined by the appended claims.

What is claimed is:
 1. A method for providing a conductive link betweena first conductive element and a second conductive element which are insubstantially the same plane, said method comprisingplacing said firstand second conductive element on a first non-conductive solid material;placing a second non-conductive solid material on said firstnon-conductive material so as to form an interface between said firstand second non-conductive material at a region between said conductiveelements; applying sufficient energy to said first and second conductiveelements to produce thermal expansion in said elements, said expansionproducing a rupturing of the interface between said first and secondnon-conductive materials so as to provide at least one fissure extendinglaterally between said first and second conductive elements along theinterface, said energy further causing a portion of at least one of saidfirst or second conductive elements to flow in said at least one fissureto provide a conductive link between said first and second conductiveelements.
 2. A method in accordance with claim 1 wherein said expansionproduces mechanical stresses at stress concentration points in saidfirst and second non-conductive materials so as to provide at least onefissure extending between said first and second conductive elements. 3.A method in accordance with claim 2 wherein said stress concentrationpoints occur generally at the lower corners of said first and secondconductive elements.
 4. A method in accordance with claim 2 wherein saidstress concentration points occur generally at the upper corners of saidfirst and second conductive elements.
 5. A method in accordance withclaim 1 wherein said conductive elements are metal elements having arelatively high coefficient of thermal expansion.
 6. A method inaccordance with claim 5 wherein said conductive elements are aluminum orcopper or alloys of aluminum or copper.
 7. A method in accordance withclaim 1 wherein said first and second non-conductive materials havedifferent mechanical strength characteristics.
 8. A method in accordancewith claim 7 wherein said first and second non-conductive materials areglass materials.
 9. A method in accordance with claim 8 wherein saidglass materials are silicon based dielectric materials.
 10. A method inaccordance with claim 8 wherein said glass materials are polymermaterials.
 11. A method in accordance with claim 1 and furtherincludingplacing a third non-conductive material on said secondnon-conductive material to form an interface between said second andsaid third non-conductive materials in the region between saidconductive elements; said sufficient energy being applied to saidconductive elements so as to produce a rupturing of the interfacebetween said second and third non-conductive materials so as to provideat least one fissure extending between said first and second conductiveelements, a portion of at least one of said first and second conductiveelements flowing in said at least one fissure to provide a conductivelink between said first and second conductive elements.
 12. A method inaccordance with claim 11 wherein the interface between said second andthird non-conductive materials is formed substantially at a plane lyingalong the plane of upper surfaces of said first and second conductiveelements and said at least one fissure extends substantially along saidplane.
 13. A method in accordance with claim 11 wherein the interfacebetween said second and third non-conductive materials is formedsubstantially at a plane lying above the plane of upper surfaces of saidfirst and second conductive elements and said at least one fissureextends upwardly from said elements to and along the plane lying abovethe plane of said upper surfaces.
 14. A method for providing aconductive link between a first conductive element and a secondconductive element which are in substantially the same plane, saidmethod comprisingplacing said first and second conductive elements on afirst non-conductive solid material; placing a second non-conductivesolid material on said first non-conductive material so as to form aninterface between said first and second non-conductive materials at aregion between said conductive elements; placing a third non-conductivesolid material on said second non-conductive material to form aninterface between said second and said third non-conductive materials inthe region between said conductive elements; applying sufficient energyto said first and second conductive elements to produce thermalexpansion in said elements, said expansion producing a rupturing of theinterface between said second and third non-conductive materials so asto provide at least one fissure extending laterally between said firstand second conductive elements along the interface, said energy furthercausing a portion of at least one of said first and second conductiveelements to flow in said at least one fissure to provide a conductivelink between said first and second conductive elements.
 15. A method inaccordance with claim 14 wherein the interface between said second andthird non-conductive materials is formed substantially at a plane lyingalong the plane of upper surfaces of said first and second conductiveelements and said at least one fissure extends substantially along saidplane.
 16. A method in accordance with claim 14 wherein the interfacebetween said second and third non-conductive materials is formedsubstantially at a plane lying above the plane of upper surfaces of saidfirst and second conductive elements and said at least one fissureextends upwardly from said elements to and along the plane lying abovethe plane of said upper surfaces.
 17. A method for providing aconductive link between a first conductive element and a secondconductive element which are in substantially the same plane, saidmethod comprising:embedding the first and second conductive a elementsin non-conductive solid material with non-conductive solid materialspanning a gap between the conductive elements, the non-conductive solidmaterial being layered to form at least one interface; and applyingsufficient energy to said first and second conductive elements toproduce thermal expansion of said elements, said expansion producing arupture of the non-conductive material extending laterally between saidfirst and second conductive elements and affected by at least oneinterface in the non-conductive solid material, said energy furthercausing a portion of at least one of said first and second conductiveelements to flow in said at least one fissure to provide a conductivelink between said first and second conductive elements.
 18. A method inaccordance with claim 17 wherein the interface along which the ruptureextends is formed substantially at a plane lying along the plane ofupper surfaces of said first and second conductive elements and saidrupture extends substantially along said plane.
 19. A method inaccordance with claim 17 wherein the interface along which the ruptureextends is formed substantially at a plane lying above the plane ofupper surfaces of said first and second conductive elements and saidrupture extends upwardly from said elements to and along the plane lyingabove the plane of said upper surfaces.